CMIP7 requirements test

[mauzey1]

Changes to enable CMOR to write CMIP7 datasets.

• The branding_suffix of variables will be tested against the branding_suffix template provided by the CV. Its components temporal_label, vertical_label, horizontal_label, and area_label will have their values validated by their corresponding sections in the CV.
• mip_era and data_specs_version will be set from the CV.
• frequency can be defined in the user input JSON or by calling cmor_set_cur_dataset_attribute prior to creating the time axis and variable that uses the axis.
• Interval values for the frequencies are acquired from the CV if the current dataset has a frequency. If the interval value used for the time axis doesn't match the interval values of the frequency used by the variable, then CMOR will throw an error.
• region can be defined in the user input JSON or by calling cmor_set_cur_dataset_attribute prior to calling cmor_write.
• Table attributes type, positive, valid_max, valid_min, ok_max_mean_abs, and ok_min_mean_abs are now acquired from the header of tables and used as the default for a table's variable entries.
• variable_title and brand_description can now be variable attributes if defined in the variable entry.
• _cmip7_option should be set in the user input JSON to enable CMIP7 global attribute checking similar to CMIP6 but without subexperiment tests. parent_mip_era will be set to CMIP7 for all datasets that are written with _cmip7_option set.
• data_archive_id has been changed to archive_id in the CV.
• host_collection has been added to the experiment_id entries of the CV.

[durack1]

@mauzey1 looks great